package expe.two;

import java.util.Scanner;

class Add {
    public static int sum;
    public static void calc (int a, int n, int d) {
        sum = n*a + n*(n-1)*d/2;
    }
}

public class Rg7190 {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int first = scanner.nextInt();
        int count = scanner.nextInt();
        int difference = scanner.nextInt();
        Add.calc(first, count, difference);
        System.out.println("和=" + Add.sum);

    }
}
